One application of the invention is in the domaine of mobile communication systems. Mobile terminals in terrestrial communication systems commonly use a .lambda./4 monopole whip antenna which provides an omnidirectional pattern in azimuth and an elevation pattern that depends upon the monopole geometry and the size of the ground plane on which it is mounted. Such an antenna has low gain and provides little discrimination between signals received directly and signals reflected from nearby objects. The interference between the direct signal and reflected signal can result in large fluctations in signal level. Normally this does not constitute a problem in terrestrial systems as there is adequate transmitted power to compensate for any reductions in signal strength. With the advent of satellite mobile communications systems, the down-link systems margins, i.e. from satellite to ground terminal, become more critical as the available transmitter power on the spacecraft is limited. Improvements in mobile terminal antenna gain and multipath discrimation can have a major impact on the overall systems design and performance.
An adaptive array antenna, consisting of a plurality of elements, can provide greater directivity resulting in higher gain and improved multipath discrimination. The directivity of the antenna can also be controlled to meet changing operational requirements. Such an antenna has however to acquire and track the satellite when the mobile terminal is in motion.
One type of the array antennas is disclosed in U.S. Pat. No. 3,846,799, issued Nov. 5, 1974, Gueguen. This patent describes an electrically rotatable antenna which includes several radially arranged yagi antennas having a common driven element. More particularly, in the array antenna of the U.S. patent, the common driven element and all the parasitic elements (reflectors and directors) are metal wires having a height of approximately .lambda./4, .lambda. being the free-space wavelength corresponding to the frequency of the signal fed to the driven element. The parasitic elements are arranged in concentric circles on a ground plane and the common driven element is at the center. Though close to .lambda./4, the heights of the parasitic elements are different, all wires located on the same circle having the same height. A pin diode connecting a parasitic element and the ground plane is made conducting or non-conducting by bias voltages applied to the diode, through a separate RF choke inductance. By rendering appropriate parasitic elements (reflectors and directors) operative, the radiation beam can be rotated about the common driven element.
While this antenna can rotate the direction of the beam electronically, it suffers from such shortcomings as narrow bandwidth, low gain, high sidelobes and highly inefficient design requiring 288 parasitic elements. Also it can rotate only in the azimuth.